This invention relates generally to EMI filter terminal subassemblies and related methods of construction, particularly of the type used in active implantable medical devices (AIMDs) such as cardiac pacemakers, implantable defibrillators, cochlear implants, neurostimulators, active drug pumps and the like, which are designed to decouple and shield undesirable electromagnetic interference (EMI) signals from an associated device. More particularly, the present invention relates to an improved EMI filter capacitor that includes various types of counter-drills, counter-sinks or counter-bores for convenient attachment of the feedthrough capacitor to the associated lead wires. The counter-drills, counter-sinks and counter-bores form a convenient well for placement of a thermal setting conductive adhesive, solder, braze material or the like.
Feedthrough terminal assemblies are generally well known for connecting electrical signals through the housing or case of an electronic instrument. For example, in implantable medical devices, the terminal pin assembly comprises one or more conductive terminal pins supported by an insulator structure for a feedthrough passage from the exterior (body fluid side) to the interior of the medical device. Many different insulator structures and related mounting methods are known for use in medical devices wherein the insulator structure provides a hermetic seal to prevent entry of body fluids into the housing of the medical device. In a cardiac pacemaker, for example, the feedthrough terminal pins are typically connected to one or more lead wires within the case to conduct pacing pulses to cardiac tissue and/or detect or sense cardiac rhythms. However, the lead wires can also undesirably act as an antenna and thus tend to collect stray electromagnetic interference (EMI) signals for transmission into the interior of the medical device. Studies conducted by the United States Food and Drug Administration (FDA), Mount Sinai Medical Center and other researchers have demonstrated that stray EMI, such as RF signals produced by cellular telephones, can seriously disrupt the proper operation of the pacemaker. It has been well documented that pacemaker inhibition, asynchronous pacing and misbeats can all occur. All of these situations can be dangerous or even life threatening for a pacemaker-dependent patient.
In prior art devices, such as those as shown in U.S. Pat. Nos. 5,333,095 and 4,424,551 (the contents of which are incorporated herein), the hermetic terminal pin subassembly has been combined in various ways with a ceramic feedthrough capacitor filter to decouple EMI signals to the equipotential housing of the medical device. As described in U.S. Pat. No. 6,999,818 (the contents of which are incorporated herein), the feedthrough capacitor can also be combined with inductor elements thereby forming what is known in the art as a multi-element low pass filter.
In general, the ceramic feedthrough capacitor, which has one or more passages or feedthrough holes, is connected to the hermetic terminal of the implantable medical device in a variety of ways. In order for the EMI filtered feedthrough capacitor to properly operate, a low impedance and low resistance electrical connection must be made between the capacitor ground electrode plate stack, and the metallic ferrule of the hermetic seal, which in turn mechanically and electrically connects to the overall conductive housing of the implantable medical device. For example, in a cardiac pacemaker, the hermetic terminal assembly consists of a conductive ferrule generally made of titanium which is laser welded to the overall titanium housing of the implantable medical device. This not only provides a hermetic seal, but also makes the ferrule of the hermetic terminal a continuous part of the overall electromagnetic shield that protects the electronics of the implantable medical device from EMI. The ceramic feedthrough capacitor is in turn, electrically and mechanically bonded to the ferrule of said hermetic terminal. Prior art feedthrough capacitors have two plate sets. There is no real polarity associated with a monolithic ceramic capacitor dielectric. However, for the purposes herein, one of the plate stacks is known as the active plate stack, which will be connected to the feedthrough lead wires and the second electrode plate stack will be known as the ground electrode plate stack, which really isn't connected to a ground, but is connected to the overall electromagnetic shield of the active implantable medical device. In other words, this is a method of shield grounding rather than earth grounding.
In the past, and in particular as described in U.S. Pat. Nos. 5,333,095 and 4,424,551, the connection between the feedthrough capacitor and the ferrule is typically performed using a thermal-setting conductive adhesive. One such material is a silver flake loaded conductive polyimide. The connection between the lead wires of the hermetic terminal and the passages or feedthrough holes of the ceramic feedthrough capacitor are typically made with solder, a thermal-setting conductive adhesive, a braze material or the like. The perimeter or diameter of the feedthrough capacitor is typically where its ground electrodes are connected (reference U.S. Pat. No. 5,333,095). Methods of holding the thermal-setting conductive material in place are well described by the prior art, in particular by U.S. Pat. No. 6,643,903 (the contents of which are incorporated herein) which describes a capture flange for convenient dispensing of materials. It is also described by U.S. Pat. No. 6,275,269 (the contents of which are also incorporated herein). Various methods of providing for leak detection are also provided as described in U.S. Pat. No. 6,566,978 (the contents of which are incorporated herein). However, the methodology to make a proper electrical connection between the lead wires and the feedthrough capacitor holes remains problematic.
Prior art feedthrough capacitors generally have a metallization or termination surface around their outside diameter or outside perimeter. This places all of the ground electrode plates in parallel and also provides for a convenient place for attachment of solder or thermal-setting conductive adhesives or the like. In a similar fashion, the inside diameter or feedthrough holes also have an inside diameter metallization surface which puts the active electrode plate set together in parallel. Various methods are known in prior art to make an electrical contact between the feedthrough lead wire and this inside diameter metallization which in turn contacts all of the electrode plates of the active electrode plate set. Application of the prior art metallization (also known as termination) on the capacitor outside diameter and also into all of the capacitor feedthrough holes is a time consuming and costly process. For a typical quadpolar ceramic feedthrough capacitor, application of the termination usually involves placing the capacitor on a mandrel and then rolling its outside diameter through a bed of a liquid silver-bearing glass frit. This glass frit is fired in place thereby conductively coupling all of the ground electrode plates in parallel. Then a vacuum pull process is used to pull metallization or termination material consisting of the same silver or palladium silver glass frit through the ID holes. This is followed by another high temperature glass firing operation. These operations are then followed by lapping or clean up operations to be sure that for small diameter feedthrough capacitors that there is no metallization left on the top or bottom surfaces that could lead to shorting out of the device. Electroplating is an alternative process to accomplish the above. While these processes tend to be very reliable, they are very expensive and time consuming.
One such methodology is described in U.S. Pat. No. 4,424,551. However, the process of injecting a material through repeated centrifuge steps and then repeated microblast cleaning steps is very time consuming, costly and tends to result in low process yields. A superior method of mounting the ceramic feedthrough capacitor is described in U.S. Pat. No. 5,333,095 wherein the capacitor is surface mounted. This has great advantages in that the ceramic capacitor itself is not subjected to undue mechanical or thermal stresses during laser weld installation of the hermetic seal subassembly and to the overall housing of the AIMD. It is relatively easy to make the perimeter or outside diameter ground attachment to the ferrule. However, for a capacitor with a flat surface with lead wires extending through its through-holes, it is problematic to make a reliable electrical connection. This is because solders, thermal-setting conductive polymers, brazes and the like tend to sit up on top of the capacitor. During re-flow operations, at high temperature these materials tend to migrate into undesirable positions. Sometimes the materials will migrate together and even short out one lead to another.
Accordingly, there is a need for improved structure methods and for making connections between a feedthrough capacitor and its associated lead wires which overcome the aforementioned difficulties. The present invention solves all the aforementioned problems and facilitates manufacturing.